How to read a Vsam file in COBOL starting from next record of the key value?

[Vijay Kamath]

How to read a Vsam file in COBOL starting from next record of the key value? For example Vsam has below:
ssss
Ttttt
uuuuu
Vvvvvv
wwwww
Xxxxx

The key that you have is vvvv. So in COBOL pgm we read the record after vvvvv that is from wwwww and process all records till ends of Vsam file. How to implement this in COBOL??

[DB2 Guy]

1. File Organization:

• Ensure the VSAM file is a KSDS (Key Sequenced Data Set), as this organization allows for direct access based on keys.

2. File Definition:

• Define the file in your COBOL program, matching the file's structure and key field:

COBOL

Code: Select all

FD VSAM-FILE
RECORDING MODE IS V
ACCESS MODE IS DYNAMIC
RECORD KEY IS KEY-FIELD.

Use code with caution, it's just for reference.

3. Open the File:

• Open the file for input or input-output:

COBOL

Code: Select all

OPEN INPUT VSAM-FILE.

Use code with caution, it's just for reference.

4. Position the File:

• Use the 

  Code: Select all

  START

   statement with the 

  Code: Select all

  READ NEXT

   phrase to position the file at the first record with a key greater than the given key:

COBOL

Code: Select all

MOVE "vvvv" TO KEY-FIELD. // Set the key value for positioning
START VSAM-FILE KEY NOT LESS THAN KEY-FIELD INVALID KEY PERFORM ERROR-ROUTINE
READ NEXT VSAM-FILE.

Use code with caution, it's just for reference.

5. Read and Process Records:

• Use the 

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  READ NEXT

   statement to read subsequent records:

COBOL

Code: Select all

PERFORM UNTIL READ-VSAM-FILE NOT EQUAL TO ZERO
PROCESS-RECORD.
READ NEXT VSAM-FILE AT END SET READ-VSAM-FILE TO ZERO.
END-PERFORM.

Use code with caution, it's just for reference.

6. Close the File:

• Close the file when finished:

COBOL

Code: Select all

CLOSE VSAM-FILE.

Use code with caution, it's just for reference.

Important Considerations:

• File Status Key: Check the file status key (FILE STATUS clause) after each READ and START to handle errors or end-of-file conditions.
• Error Handling: Implement appropriate error handling routines for invalid keys or other file access issues.
• Record Processing: Replace PROCESS-RECORD with your actual record processing logic.
• Key Field Size: Ensure the KEY-FIELD is defined with the correct size to accommodate the VSAM file's key length.
• Access Mode: Using DYNAMIC access mode allows for both sequential and direct access to records.

Additional Notes:

• If you need to read records starting from the first occurrence of the key value, use READ NEXT directly without the START statement.
• For more complex scenarios, consider using the READPREV statement (if available in your COBOL implementation) to read records in reverse order.

[Bravaneove]

Is there a specific reason for choosing DYNAMIC access mode, and would the implementation differ if the VSAM file is an alternate index?

[Robert Sample]

START only works with DYNAMIC or RANDOM access mode -- you cannot use it with sequential.  Conversely, READ NEXT won't work for RANDOM access mode.